(1) Field of the Invention
The present invention relates to an aqueous coating resin composition. More particularly, the invention relates to an aqueous coating resin composition useful for forming a coating directly on a tinplate, an aluminum plate, a treated steel plate or the like, or on an undercoating layer of a phenol-epoxy or epoxy-amino paint, which is formed on such metal substrate, particularly the inner surface of a metal can or can closure.
(2) Description of the Prior Art
For metal cans, there have heretofore been used a phenol-epoxy or epoyx-amino paint as a base coat paint to be applied on the inner surface and a thermoplastic copolymer composed mainly of vinyl chloride and vinyl acetate as a topcoat-forming material. When the base coat alone is applied, the flavor of the content is degraded, and when only the topcoat of a copolymer composed mainly of vinyl chloride and vinyl acetate is formed, the adhesion to the metal substrate and the resistance are insufficient. Since this copolymer is readily decomposed under heat, the temperature range for the baking treatment is very narrow. Furthermore, since conventional topcoat-forming paints are of the non-aqueous type and contain considerable quantities of organic solvents, pollution of the working atmosphere and other problems are caused and the use of these paints involves a risk of occurrence of a fire.
As means for solving these problems, there have been proposed various aqueous coating compositions to be used instead of the conventional vinyl chloride-vinyl acetate paints. For example, the specification of U.S. Pat. No. 4,021,396 discloses an aqueous coating composition obtained by neutralizing an epoxy resin and an acrylic copolymer formed by copolymerizing 0.5 to 10% of an unsaturated carboxylic acid with other specific monomers, with ammonium or an amine. From experiments made by us, it was found that this aqueous coating composition is poor in the storage stability, particularly at a high temperature (about 50.degree. C.) and gelation is readily caused at such high temperature, and that at the baking step, there is caused a problem that the physical properties of the resulting coatings vary widely depending on the difference of the baking temperature.
Japanese Patent Application Laid-Open Specification No. 1228/78 proposes a paint formed by grafting an unsaturated carboxylic acid-containing monomer to the aliphatic skeleton of an epoxy resin, neutralizing a mixture of this graft polymer and a carboxylic acid-modified functional addition polymer with ammonium or an amine and dispersing the neutralization product in an aqueous medium. Furthermore, Japanese Patent Application Laid-Open Specification No. 1285/78 discloses an improvement of the above-mentioned technique, in which the epoxy group of an epoxy resin is reacted with a stopping agent to improve the resistance to hydrolysis.
The proposals made in these Japanese Patent Application Laid-Open Specifications still involve problems to be solved. For example, since it is necessary to effect graft polymerization, expensive and dangerous benzoyl peroxide or a corresponding free radical initiator should be used in large quantities. Moreover, the molecular weight of the addition polymer is reduced, resulting in degradation of the physical properties of the resulting coatings. Therefore, there are caused various disadvantages with respect to physical properties, adaptability to operations and manufacturing and running costs. Still further, scattering of properties in products is readily caused according to reaction conditions adopted for the graft polymerization.
The most important role of a composition for coating the inner surface of a metal can is to sanitarily protect the content. If dissolution of components of the coating into the content is advanced at the sterilizing step or during long-time storage, the flavor of the content is degraded and a sanitarily undesirable phenomenon is caused by the extracted components. Accordingly, the coating formed on the inner face of the metal can should have such a characteristic that dissolution of components of the content into the content should be reduced to a level as low as possible under treatment conditions which the metal can actually undergoes. (Ordinarily, dissolution of components of the coating into the content is evaluated depending on the ratio of extraction of the components in water from the coating.) All of the aqueous coating compositions according to the above-mentioned proposals are still insufficient in this point. It is therefore a primary object of the present invention to provide an aqueous coating composition which is excellent in the stability and can give coatings having excellent physical properties and in which dissolution of components of the coating into the content can be controlled to a very low level.
From experiments made by us, it was found that in an aqueous coating composition comprising an acrylic resin and an epoxy resin, if the molecular weight of the epoxy resin used is low, the ratio of extraction of the components in water from the resulting coating tends to increase. In this case, the epoxy group of the low-molecular-weight epoxy resin chemically reacts in the aqueous medium during storage, resulting in increase of the viscosity or occurrence of gelation. The coating formed by using such composition having an increased viscosity or such gelled composition is inferior in various physical properties. On the other hand, when a high-molecular-weight epoxy resin having a number average molecular weight exceeding 1,400 is employed, the water extraction ratio is considerably reduced, but the level of the water extraction ratio is not so low as the level attainable by the conventional vinyl chloride copolymer paint. Although chemical reaction of the epoxy group of the high-molecular-weight epoxy group during storage is reduced, the compatibility of the epoxy resin with the acrylic resin is poor and the epoxy resin tends to separate from the acrylic resin during storage. Furthermore, in the resulting coating, whitening is readily caused owing to this poor compatibility, and a coating having satisfactory physical properties cannot be obtained.